Regulations for animal research require humane treatment of laboratory animals. Because animals cannot articulate pain, humane principles mandate that scientists provide the same analgesia that humans need for similar surgery. Indeed, an increasing body of evidence has shown that pain receptors in rodents and humans are virtually identical and that the sensation of pain would be highly comparable (Cowan, et al. Eur J Pharmacology, 507: 87-98, 2005). A second principal is that regulations for animal research be internationally harmonized so that pharmaceutical studies have the same scientific basis in every country.
The U.S. Department of Agriculture (USDA) regulates animal research in the US (Animal Welfare Act and Amendments, (7 U.S.C. 2131 et seq APHIS, USDA). With one exception, USDA regulations for animal welfare are similar to regulations in the Europe, Japan, and other industrialized nations. In the US, mice and rats are not considered to be “animals.” This exception allows scientists in the US to provide lower and less expensive standards of analgesia. Studies have shown that at least 80% of scientists fail to provide adequate post-surgical analgesia for rodents (Richardson, et al. Altern Lab Anim 33: 119-127, 2005). Not surprisingly, the exception is the basis of anti-USDA lawsuits from animal welfare organizations.
It is estimated that more than 60 million mice and rats are used yearly in medical and toxicology research, with the majority of rodents being used in the US. The loophole in USDA regulations regarding the post surgical care of rodents may play a role in these use patterns. Many observers predict that US regulations will soon be come harmonized with international regulations and that rodents will be afforded greater protection based on scientific concerns and humane principles.
A low cost, safe and easy to use analgesic for rodent surgery could meet many of the concerns that have been articulated by scientists in the US. Scientists have defended their reluctance to use analgesia for rodents based on the expense of post-surgical care, concerns that analgesia could interfere with the outcome of an experiment, and the difficulty of handling small animals.
The cost of post-surgical care can be significant and the management of analgesia requires expensive labor resources. Drugs such as aspirin and ibuprofen must be given orally (PO) at 4-6 hour intervals. Morphine and buprenorphine have to be injected subcutaneously (SQ) or intraperitonealy (IP) at 6-8 hour intervals. Injured animals do not eat or drink on normal schedules, so food and water cannot be effectively spiked with drugs. Whether post-surgical analgesia affects the outcome of an experiment can be determined. Scientists routinely state their concerns that affects can occur (Karas Ariz., Lab Animal 35: 38-45, 2006). Frequently, these statements are made without experimental proof (Silverman J, Lab Animal, 30 (3): 21-26, 2001). It is highly doubtful that among the many choices of analgesic drugs, one or more drug could not be found to humanely treat the animal and not interfere with the experiment.
The difficulty in providing analgesia to small animals is perhaps the most significant barrier to rodent care. It is difficult to grab and hold a mouse and rat without disturbing the surgical wound, injuring the spine, and without causing more pain. Once restrained, animals resist PO therapy. SC and IP injections cause pain. IP injections cause infections and death if the needle penetrates sensitive tissue (Coria-Avila, et al. Lab Animal 36: 25-30, 2007.) A chronic delivery form of analgesia for rodent surgery would have significant benefits.
Several efforts have been made to use chronic drug-delivery systems such as infusion pumps and biodegradable scaffolds for laboratory and companion animal medicine. Alzet osmotic pumps and other swellable core technologies have become a standard method for delivering constant levels of drugs to animals for research (Guarnieri, et al. J Neuroscience Methods, 144: 147-152, 2005). Additional efforts have included the use of transderinal patches (Bohme Clin Rheumatol, 21: S13-S16, 2002), and food-based drugs. Transdermal patches have limited use because rodents and companion animals scratch and remove skin patches. Food-based analgesia has not been widely studied because post-surgical animals frequently have erratic eating patterns and because of the costs associated with adding drugs to standard chows.
An implantable, rapid-release analgesic would significantly reduce barriers to humane post surgical animal care. However, as demonstrated by the examples, most controlled release systems do not provide the necessary blood levels of analgesics required. Biodegradable drug implants would eliminate the safety concerns about handling small animals after surgery. Cholesterol implants have been used to provide sustained release of macromolecules, but not for the short-term delivery of analgesia (U.S. Pat. No. 4,452,775 to Kent. Lipospheres for controlled delivery of substances have been described in U.S. Pat. No. 5,188,827 to Domb. Liposomes, solid lipid nanoparticles, oily suspensions, and fatty acids for sustained release parenteral formulations and implants have been described in U.S. Pat. No. 5,137,874 to Cady. Waxes, lipid microspheres, and lipid implants have been described as sustained-release vehicles, as reviewed by Mohl S, The Development of a Sustained and Controlled Release Device for Pharmaceutical Proteins based on Lipid Implants. Dissertation, University of Munich, 2004. In the majority of cases, the lipid and other biodegradable carrier such as PLGA copolymers (Shaw et al, 1993) are designed for the long-term sustained release of drugs and vaccines (U.S. Pat. No. 4,164,560 to Folkman and Langer.
Several exceptions can be noted to the focus of biodegradable lipid scaffolds and long-term drug delivery. Lidocaine and bupivcaine biodegradable scaffolds such as SABER and lipospheres have been designed for 2-3 hour duration of anesthesia for surgical wounds (Hersh, et al. Anesth Prog. 39(6): 197-200, 1992; Toongsuwan, et al. Int J Pharmaceutics 280: 57-65, 2004). Liposomal preparations of morphine and oxymorphone have been tested in mouse and rat pain models (Grant, et al. Anesthesia and Analgesia 79: 706-709, 1994; Krugner-Higby, et al. Comparative Medicine 53: 270-279, 2003; Clark, et al. Comp Med, 54: 558-563, 2004). Liposomes, however are difficult to prepare and must be used fresh. Morphine is a more highly regulated drug and provides less analgesic efficacy compared to buprenorphine.
Buprenorphine esters can act as buprenorphine pro drugs in sesame, castor, cottonseed, peanut or soybean oil solutions injected into rat muscle (IM). The analgesic effect for sesame oil solutions could be measured for approximately 3 days (Liu, et al. Anesth Analg. 102: 1445-1451, 2006; J Pharm Pharmacol. 58(3): 337-34, 2006). The later studies contrast with our inability (Table 5) to detect buprenorphine in blood following SC injections of buprenorphine in oleic acid, an oil with properties highly similar to the oils used by Liu and coworkers.
Pontani, et al., Pharmacology Biochemistry & Behavior 18: 471-474, 1983 and Xenobiotica, 15, 287-297, 1985 describe a 50 mg cholesterol implant designed for the sustained release of buprenorphine for long-term therapy to prevent opiate seeking behavior. The 50 mg implant releases buprenorphine at a uniform rate (first order kinetics) for at least 12 weeks. While this release profile is suitable for sustained delivery of anti-addiction medication, it is highly unsuitable for short-term post surgical analgesia. The long-term presence of a foreign body and drug could have significant impact on research on vaccines, neurodegenerative disease, cancer, immunity-related chronic diseases such as diabetes, and other diseases.
It is therefore an object of the present invention to provide a controlled release formulation for delivery of analgesics to laboratory and research animals, especially small rodents.